Process of fabricating a composite zinc printing plate

ABSTRACT

In all forms of the invention at least one heat absorbing aluminum backing strip has one side prepared for bonding zinc thereto. The strip is then moved first through a pool of molten zinc and then through a slot in a cooling die. The slot in the die is wider than the thickness of the strip. Zinc from the pool is caused to flow controllably into the die slot and to freeze on the prepared face to form a zinc coating thereon as the strip approaches and passes through the die. The die is of a nonsticking short-throat type so as to produce a smooth finish on the coating before complete freezing occurs. The side of the strip which is not to receive a coat of zinc is shielded from the molten zinc. In one form of the invention, shielding from the zinc is accomplished by placing two aluminum strips back to back as they pass through the zinc melt and the die. In another form, the back of the aluminum strip is shielded by omitting to clean that side of the strip and/or applying a suitable zinc-repellant substance such as a film of oil or the like. In another form of the invention the back of the aluminum strip engages part of the crucible containing the melt, to effect shielding.

United States Patent 72] Inventor John F. Clarke Attleboro, Mass.

[21] Appl. No. 691,329

[22] Filed Dec. 18, 1967 [45] Patented Mar. 9, 1971 [73] Assignee Texas Instruments Incorporated Dallas, Tex.

[54] PROCESS OF FABRICATING A COMPOSITE ZINC PRINTING PLATE 5 Claims, 4 Drawing Figs. [52] US. Cl 164/86, 164/2,101/401.1 [51] Int. Cl. ..1 322d 11/0 B41d 3/00 [50] Field ofSearch 164/2, 86,

91,95,102,140, 141,275, 332, 64, 283; 101/401.l (US), 395 (US) [56] References Cited UNITED STATES PATENTS 3,470,939 10/1969 Coad l64/283X 1,863,809 6/1932 Hopkins et a1. l64/275X 2,257,143 9/1941 Wood 101/401.1UX 2,301,902 11/1942 Merle 164/86 2,692,411 10/1954 Brennan 164/86X 3,136,672 6/1964 Prongay 10l/395X FOREIGN PATENTS 1,454,398 4/1966 France 9/197 812,380 4/1959 Great Britain 164/2 Primary Examiner-Charles W. Lanham Assistant Examiner-R. Spencer Annear Attorneys-Harold Levine, Edward J. Connors, J r., James P.

McAndrews, John A. l-laug and Gerald B. Epstein ABSTRACT: In all forms of the invention at least one heat absorbing aluminum backing strip has one side prepared for bonding zinc thereto. The strip is then moved first through a pool of molten zinc and then through a slot in a cooling die.

The slot in the die is wider than the thickness of the strip. Zinc from the pool is caused to flow controllably into the die slot and to freeze on the prepared face to form a zinc coating thereon as the strip approaches and passes through the die.

The die is of a nonsticking short-throat type so as to produce a smooth finish on the coating before complete freezing occurs. The side of the strip which is not to receive a coat of zinc is shielded from the molten zinc.

In one form of the invention, shielding from the zinc is accomplished by placing two aluminum strips back to back as they pass through the zinc melt and the die. In another form, the back of the aluminum strip is shielded by omitting to clean that side of the strip and/or applying a suitable zinc-repellant substance such as a film of oil or the like. In another form of the invention the back of the aluminum strip engages part of the crucible containing the melt, to effect shielding.

PRKMJESS QT TAhfIATlNG A (IQWOSETE Zlbltll PRHNTTNG liLA-TE The invention is an improvement upon those disclosed in the U.S. Pat. application of Robert J. Russell and Russell E. Miller, Ser. No. 593,535 filed Nov. 10, i966 (attorneys file 283 i) now abandoned in favor of a continuation application, Set. No. 824,718, filed Apr. 9, l96 9;also those disclosed in the US. Pat. application of Brian C. Coad, Ser. No. 506,743, fried Nov. 8, 1965 (attorneys file 2776) now U.S. Pat. No. 3,470,939, issued Oct. 7, 1969, the aforementioned applications and patent being assigned to the assignee of the present application.

The present invention, as in the Russell-Miller application, relates in general to the reduction in etched zinc printing plates of so-called pimpling, which results in a grainy printed copy. As set forth in the Russell-Miller application, a major difficulty encountered in the use of engravers zinc plates is the random formation of the so-called pimples in the etched depressions of the printing surfaces of the plates. These pimples are small pointed protrusions, which must be mechanically removed by a hand tool or machine before a plate canbe used on a printing press. if not removed, the protrusions print unwanted dots on the paper and produce an unattractive mottied appearance. There is considerable cost involved in removing such pimples and there is also a danger of spoiling a plate to the extent that it must be scrapped. Pimpling appears to be caused by concretions caused by the iron or other impurities in the zinc. These concretions are not attached by the etchant used to etch the plate and therefore stand out to print unwanted dots in printed copy. I

The Russell-Miller application suggests that pimpling can be minimized by reducing the iron impurity in the zinc used to make the plate, but this has its limits because of the cost involved. Therefore they suggest cooling of zinc by a batch casting process which reduces pimpling near the printing surface. The cooling appears to obviate the concretions due to any iron content. The greater the amount and rate of heat abstraction the better. i employ a continuous process which has a number of advantages, i.e., at high speed and low cost an aluminumbaked zinc printing plate is produced wherein an aluminum backing during processing acts along'with a finishing and cooling die very rapidly to abstract large amounts of heat from the zinc. The rate of heat abstraction is on the order of one or more magnitudes greater than the rate heretofore accomplished, thereby reducing pimpling of the plate upon etching, not only near the surface, but more effectively throughout the entire depth of zinc layer and down to the aluminum backing layer.

A common but not the only zinc alloy for making zinc plates according to the invention consists nominally of approximately 0.06 to 0.14 percent aluminum (Al), 0.00 to 0.20 percent magnesium (Mg), the balance zinc (Zn) with trace amounts of impurities such as cadmium, copper, lead and iron as above mentioned. These trace impurities, and more particularly the traces of iron, are believed to be the major if not the only cause of pimpling. As mentioned, by using a zinc alloy having a low iron content some improvement is possible. But more importantly, by increasing the rate of the heat removal during cooling of the cast zinc, and effecting removal across the entire thickness of the zinc layer, the number of pimples at or near the printing surface of the zinc plate can be effectively reduced to a greater depth. Thus an aluminum-backed zincfaced plate is produced in which pimpling, if any, is innocuous regardless of etch depth. Other objects and features will be in part apparent and in part pointed out hereinafter.

H0. l is a vertical section illustrating one form of the inven- Referring now more particularly to FIG. ll, there is shown at numeral l a part of a suitable crucible for containing a melt 3. Heating means is diagrammatically indicated at 5. The melt is composed of any desired zinc printing-plate alloy, one formula for which has been mentioned above.

Connected to the bottom '7 of the chamber 1 is a rectangularly slotted cooling die 9. The length L and width W of its slot ii are indicated in FIG. 2. The slot includes a comparatively short upper inlet or throat portion 13} (FIG. l) which has substantially parallel sides, thus producing a rectangular form of the die throat having the desired cross section. The lower or outlet portion of the die 9 is downwardly flared as shown at 15. Contained in the die around the throat 13 and flare 15 are passages 17 for circulation of a suitable coolant such as water. It is to be noted that the die 9 is cooled throughout its length, at its ends and throughout its depth, including the throat l3 and the flare 15.

Projecting downward into the melt 3 is a guide 19 composed of a suitable heat-resistant or refractory material. Its outlet 21 may or may not be positioned below the upper surface of the melt 3, the latter case being shown in FIG. 1. The passage 23 through the nozzle 19 is of a proper cross section to admit and guide two aluminum strips 25 located back to back for downward movement through the melt 3. An appropriate thickness of each strip 25 is, for example, 13 mils. Each of these is to become a backing layer for a zinc layer of the printing plate ultimately to be made. The width W of the inlet throat portion 13 of the slot is sufficient to provide for passage as, for example, at a 19 mil or more spacing of each outer face of the two strips 25. On the other hand, the length L of the die slot inlet portion 13 closely equals the widths of the strips 25.

At numeral 27 is shown a stopper, gate or well of heat-resistant or refractory material. This gate surrounds and extends below the lower end of the nozzle 19, its lower margin 29 being near but spaced from the container bottom 7. Thus, what may be referred to as a coating chamber 31 is provided around the strips 25 in their movement from the nozzle 19 down through the die 9. The gate 27 is vertically adjustable. Suitable means known in the art may be employed for adjustment, this being illustrated by the double dart 32. Thus the rate of flow of the melt ill-into the coating chamber 3i may be controlled. The liquid level carried in this coating chamber 31 may be at (as shown) or below that in the remainder of container l.

in operation the two strips 25 at room temperature, for ex ample, are moved downward through the zinc melt in the coating chamber 3i; but before entering the guidew and melt 3, their outer surfaces are well cleaned, for example, by skivers 35. As the strips leave the nozzle 19 they pass through the coating chamber 311 in which the heat flow from the molten zinc to the cooler strips in one direction is as illustrated by the horizontal darts 35. The result is a partial but considerable freezing effect bonding the melt on the clean strip surfaces. Thus a semifrozen partial coating 37 of zinc builds up on each strip as it descends into the die 9. But the buildup does not infill the spaces between the strips and the straight throat l3. The remaining spaces receive inflows of the zinc melt as at 39 The coolant circulating in the die 9 additionally rapidly abstracts heat in an opposite direction as shown by the darts ll. This is carried away by the coolant. This semifreezes the sheetlike inflows to complete the thicknesses of the zinc layers 37 on each backing strip as indicated. This thickness of each complete zinc layer may be at final gauge (for example 19 mils) on each strip 25 or it may be thicker by properly dimensioning the width W. The downward speed of the aluminum strips 25 ft./min., for example) is great enough and the distance between the skivers 33 and the top of the melt 3 (2 inches for example) is short enough so that the freshly cleaned aluminum does not become substantially recontaminated before bonding. Similarly the distance between the surface of the melt 3S and the exit of the die 9 (6 inches for example) is short enough to achieve extremely high freezing rates. in the cornparatively short time of passage through the upper throat portion 13 of the die a completely hard freeze does not occur thus minimizing sticking. The cooled flaring die portions 43 complete the zing. The result is a smooth and accurate solid zinc facing 45 at or near final gauge on the outer surface of each of the aluminum strips devoid of surface blemishes. The very rapid rate of cooling from opposite sides of the zinc layers greatly minimizes pimpling throughout their thicknesses. Draw rolls 4'7 pull the zinc-coated strips through the process. As illustrated at 49, the unconnected coated strips may be separated, this being done by any conventional separating means. From each strip are cut out printing plates of whatever size desired. it is to be understood that if the zinc is cast on the strips 25 at greater than finished gauge then resulting composites may be rolled down to that gauge.

In FlG. 3 is shown in fragmentary form a modification of the H6. 1 form in which the zinc is cast from the melt on a single aluminum backing strip 25. Like numerals designate like parts. In this case, the back face 17 of strip 25 engages one side of the die 9. The back face is left unclean and touches one side of throat 13 so that no zinc bonds to it. The result shown is a single composite aluminum-backed zinc-faced strip.

H6. 4 shows another modification in which the crucible i and the die 9 are so constructed that the back face 17 of strip 25 is backed by and slides along onewall 18 of the crucible. Thus if perchance an otherwise unclean back face 17 might have clean spots, these will not be reached by the zinc for bonding. in this construction, the gate is in the form of a wall 30 sliding in groove means 3:6 in the crucible. A layer of suitable flux 34 is floated on the top of the zinc melt in the coating chamber 31. An example of the flux that may be used in Alcoa flux No. 67 comprising zinc chloride, ammonium chloride, sodium fluoride in a 44:5:1 ratio mixed with water. When floated on the zinc melt the water disappears leaving the active flux constituents. This flux reacts with the face 38 of strip 25 before this face enters the melt 3, thus preparing it for bonding of the zinc. In this case, precleaning may not be required, depending upon the amount of any contaminants on face 38.

l have further found that the invention results in the rate of heat removai (Btu/lb/sec) from the zinc layer 43 is on the order of one or more orders of magnitudes greater than the maximum rate specified in said Russell-Miller application, which is to say times or more faster. Moreover, the heat removal is accomplished across the entire thickness of each zinc layer $3 because heat is being abstracted from opposite sides of the zinc layer. As a consequence, the reduction in the pimpling effect is great and effective across the entire thickness of the zinc, rather than simplyon one side. Therefore various depths of etching may be accommodated without danger of entering any substantially pimpling zone.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

3. The process of manufacturing a composite zinc printing plate from a metal backing layer and zinc containing trace impurities which normally cause pimpling effects when used to form an etched printing face, comprising moving a continuous strip of said backing layer through a melt of the zinc and exposing one face of thebacking layer to the zinc melt, abstracting heat from the melt for bonding of the zinc to said face at the interface with the backing layer, the heat being abstracted by the backing layer in a first direction toward the backing layer so as to partially freeze some of the melt and form a partially frozen zinc layer having one face bonded to the backing layer at the interface between the melt and the backing layers, and continuing the passage of the backing layer with the partially frozen zinc layer thereon adjacent a cooling die to abposite having throughout the thickness of the zinc layer no more than harmless pimpling effects of the impurities throughout the thickness thereof.

2. The process of manufacturing an improved composite zinc printing plate from a backing layer and zinc containing tract impurities which normally cause pimpling effects when used to form an etched printing surface, comprising moving a continuous length ofra heat-absorbing aluminum strip through a melt of the zinc and exposing one face to the zinc melt for bonding of the zinc to said face, abstracting heat by the aluminum layer from the melt in a direction toward the aluminum layer to at least partially freeze some of the melt and partially form a zinc layer having one face bonded to the aluminum layer at the interface between them, continuing the passage of the composite of the aluminum layer with the partially frozen zinc layer thereon adjacent a cooling die which defines a space with respect to the partially formed zinc layer, causing zinc melt to flow into said space to complete the zinc layer and form a finish thereon adjacent said die, and abstracting heat by said die in a direction toward said die and away from said aluminum layer, whereby the pimpling effects of the impurities are reduced substantially throughout the thickness of said zinc layer.

3. The process of making a composite zinc printing plate comprising engaging two aluminum strips at their backs, cleaning, preparing their fronts for bonding with melted zinc, moving said strips through a zinc melt and then through a cooling die having a comparatively short throat providing spaces relative to the fronts of the strips for the reception of some of the zinc melt and having a flaring portion, said die also including cooling means around the throat and flaring portion, whereby the zinc freezes to the opposite fronts of the strips first by abstraction therefrom by the aluminum strips and then by abstraction of heat by the die, separating the resulting composites formed by the bonded strips and the zinc layers thereon and forming individual printing plates from each of the two composites.

4. The process of making a composite printing plate comprising moving an aluminum strip through a zinc melt on which is floated a fluxing material, protecting one side of the aluminum strip from contact by the zinc melt but exposing the other side thereto, said aluminum strip entering the melt through the layer of flux thereon, continuing movement of the strip through a cooling die having a comparativ ely short throat a portion of which is spaced from the unprotected side of the aluminum strip and having a flaring portion, said throat and flaring portion being cooled, whereby initial freezing of the zinc melt on the aluminum strip is caused by heat abstraction from the melt by the aluminum strip and secondly by heat abstraction from the other side of the zinc layer by the die, and forming individual plates from the strip after exit from the die.

5. A process of manufacturing a composite zinc plate from a metal backing layer and molten zinc containing trace impurities which cause pimpling effects when the zinc is solidified, comprising moving a continuous length of said metal backing layer which comprises a pair of metal backing layers arranged in facing engaging relationship, initially through a melt of the zinc to effect bonding of semifrozen zinc on opposed outwardly facing surfaces thereof as a result of heat abstraction by said pair of backing layers from said melt, then moving said backing layers adjacent a cooled forming die to solidify additional zinc onto said opposed outwardly facing surfaces as a result of heat abstraction by said cooling die from said zinc melt, thereby preventing the formation of any substantial number of pimpling concretions of the impurities, and subsequently separating the resulting composite structures formed by each individual backing layer and the zinc solidified on the respective outwardly facing surfaces thereof. 

2. The process of manufacturing an improved composite zinc printing plate from a backing layer and zinc containing tract impurities which normally cause pimpling effects when used to form an etched printing surface, comprising moving a continuous length of a heat-absorbing aluminum strip through a melt of the zinc and exposing one face to the zinc melt for bonding of the zinc to said face, abstracting heat by the aluminum layer from the melt in a direction toward the aluminum layer to at least partially freeze some of the melt and partially form a zinc layer having one face bonded to the aluminum lAyer at the interface between them, continuing the passage of the composite of the aluminum layer with the partially frozen zinc layer thereon adjacent a cooling die which defines a space with respect to the partially formed zinc layer, causing zinc melt to flow into said space to complete the zinc layer and form a finish thereon adjacent said die, and abstracting heat by said die in a direction toward said die and away from said aluminum layer, whereby the pimpling effects of the impurities are reduced substantially throughout the thickness of said zinc layer.
 3. The process of making a composite zinc printing plate comprising engaging two aluminum strips at their backs, cleaning, preparing their fronts for bonding with melted zinc, moving said strips through a zinc melt and then through a cooling die having a comparatively short throat providing spaces relative to the fronts of the strips for the reception of some of the zinc melt and having a flaring portion, said die also including cooling means around the throat and flaring portion, whereby the zinc freezes to the opposite fronts of the strips first by abstraction therefrom by the aluminum strips and then by abstraction of heat by the die, separating the resulting composites formed by the bonded strips and the zinc layers thereon and forming individual printing plates from each of the two composites.
 4. The process of making a composite printing plate comprising moving an aluminum strip through a zinc melt on which is floated a fluxing material, protecting one side of the aluminum strip from contact by the zinc melt but exposing the other side thereto, said aluminum strip entering the melt through the layer of flux thereon, continuing movement of the strip through a cooling die having a comparatively short throat a portion of which is spaced from the unprotected side of the aluminum strip and having a flaring portion, said throat and flaring portion being cooled, whereby initial freezing of the zinc melt on the aluminum strip is caused by heat abstraction from the melt by the aluminum strip and secondly by heat abstraction from the other side of the zinc layer by the die, and forming individual plates from the strip after exit from the die.
 5. A process of manufacturing a composite zinc plate from a metal backing layer and molten zinc containing trace impurities which cause pimpling effects when the zinc is solidified, comprising moving a continuous length of said metal backing layer which comprises a pair of metal backing layers arranged in facing engaging relationship, initially through a melt of the zinc to effect bonding of semifrozen zinc on opposed outwardly facing surfaces thereof as a result of heat abstraction by said pair of backing layers from said melt, then moving said backing layers adjacent a cooled forming die to solidify additional zinc onto said opposed outwardly facing surfaces as a result of heat abstraction by said cooling die from said zinc melt, thereby preventing the formation of any substantial number of pimpling concretions of the impurities, and subsequently separating the resulting composite structures formed by each individual backing layer and the zinc solidified on the respective outwardly facing surfaces thereof. 